1. Technical Field
The present invention relates to a signal processing apparatus which performs signal processing on an input signal to produce an output signal, and more particularly to a technique for reducing the noise level.
2. Background Art
As one of techniques for reducing the noise level, there is a noise gate which attenuates a signal during a time period containing only noise components. Noise gates include a digital noise gate in which a digital signal is attenuated in a digital portion to and from which the signal is input and output, and an analog noise gate in which an analog signal is attenuated in an analog portion to and from which the signal is input and output.
With respect to an analog noise gate, there is a technique disclosed in JP-A-2003-52096. JP-A-2003-52096 discloses a noise gate apparatus including: a head amplifier which amplifies an audio signal; a noise gate which blocks the amplified audio signal during a time period when the amplitude value is equal to or lower than a threshold; and a parameter conversion unit which sets the threshold corresponding to the gain (amplification factor) of the head amplifier, in the noise gate.
In a device in which a severe restriction is imposed on power consumption, such as a portable telephone, a configuration is preferably employed in which the amplitude of an audio signal is set as low as possible in front of an amplifier, and largely increased in the amplifier. When this configuration is employed, the digital noise gate cannot reduce noise components which are mixed in a DAC (Digital to Analog Converter) in a stage subsequent to the digital portion and the amplifier. In the analog noise gate, such noise components can be reduced, but it is difficult to specify a time period containing only noise components with high accuracy based on a signal with which such noise components are mixed. Accordingly, the noise level cannot be reduced with sufficiently high accuracy.